1. Field of the Invention
The present invention relates to an ASK modulator; particularly, the invention relates to an ASK modulator used for an electronic toll collection (ETC) system.
2. Description of the Related Art
FIG. 5 shows a conventional amplitude shift keying (ASK) modulator. In an ASK modulator 1 shown in FIG. 5, the drain of a field effect transistor 2 (FET 2) having a negative pinch-off voltage of -1 V is connected to a power-source terminal 4 via an inductor 3. The drain is also connected to a modulation-signal output terminal 6 via a capacitor 5. A positive voltage is applied to the power-source terminal 4. The source of the FET 2 is grounded via a resistor 7 and a capacitor 8 connected in parallel. The gate of FET 2 is connected to a carrier-signal input terminal 9. A data-signal input terminal 10 is grounded via resistors 11 and 12 which are connected in series, and a node where the resistors 11 and 12 are connected is connected to the gate of the FET 2.
FIGS. 6A-6C show waveforms of signals that are inputted to the carrier-signal input terminal 9 and the data-signal input terminal 10 and a signal that is outputted from the modulation-signal output terminal 6. Referring to these waveforms, the operation of the ASK modulator 1 is described below. The amplitudes of the individual signals in the figures are not necessarily drawn to scale. As an example, for reference, the amplitude of the carrier signal may be smaller than that of the data signal, and the amplitude of the modulation signal may be greater than that of the carrier signal.
First, a carrier signal with a sine waveform as shown in FIG. 6A is inputted to the carrier-signal input terminal 9. On the other hand, a digital data signal as shown in FIG. 6B is inputted to the data-signal input terminal 10. The potential of the data signal is assumed to be 0 V at a high (H) level, and is assumed to be -5 V at a low (L) level. As a result, the data signal with the carrier signal overlapped is inputted to the gate of the FET 2.
When the data signal is at the H level, the voltage between the gate and the source of the FET 2 is higher than the pinch-off voltage, thereby causing the FET 2 to perform an amplifying operation. The carrier signal is thus amplified, and the amplified signal is outputted from the modulation-signal output terminal 6. In contrast, when the data signal is at the L level, since the voltage between the gate and the source of the FET 2 is lower, the FET 2 does not perform the amplifying operation. Therefore, the modulation-signal output terminal 6 outputs no signal.
Thus, as shown in FIG. 6C, depending on the condition, the modulation-signal output terminal 6 either outputs the carrier signal amplified in relation to the data signal or does not output any signal. It is apparent that this is attributable to ASK-modulation (amplitude modulation) wherein the data signal digitally modulates the carrier signal. In this way, the ASK modulator 1 implements the ASK-modulation operation.
With the ASK modulator 1, to perform the modulation operation, however, at least an L-level potential of the data signal must be the same as or lower than the pinch-off voltage of the FET 2, that is, it must be a negative potential. This requires a negative-voltage power source for supplying the negative potential, in addition to the power source for supplying the positive voltage that is applied to the power-source terminal 4. This produces problems in that the provision of the additional negative power source makes it difficult to implement miniaturization of the ASK modulator 1 and of a module using it, and makes cost reduction difficult as well.